During the packaging of an integrated circuit device, it is necessary to connect the tiny electrical connection pads of the integrated circuit device (fabricated on a portion of a silicon wafer called a die) to the leads of a surrounding lead frame, the leads being more suited to electrical connection with other electronic devices.
Electrical connection of the pads on the die to the leads of the leadframe is accomplished by bonding a fine metal wire to each pad on the die to a corresponding lead on the surrounding lead frame. On each pad of the die, a "ball bond" is formed to bond the wire to the pad. On each corresponding lead of the lead frame, a "crescent bond", for example, is formed that bonds the wire to the lead. The formation of the crescent bond will now be described with reference to FIG. 1.
During bonding of the wire 10 to a lead 12, the wire 10 is fed through a capillary 14, i.e., a hollow metal tube, as the capillary 14 is moved from the ball bond (not shown) of the wire 10 on a pad (not shown) to the corresponding lead 12 of the lead frame (not shown). When it reaches a desired target position over the lead 12, the capillary 14 is pressed firmly against the lead 12, and can be heated and/or vibrated at high frequency. This causes the wire 10 to flatten, spread out, and metallurgically bond to the lead 12, thereby forming a crescent bond or crescent 16, the crescent including a portion that is bonded to the lead 12. The bond so-formed provides both an electrical and mechanical connection between the wire 10 and the lead 12. Both the shape and position of the crescent 16 with respect to the lead 12 indicate and affect the quality of the electrical and mechanical connection of the wire 10 to the lead 12. The pressing, vibrating, and/or heating of the capillary 14 on the lead 12 (using a "thermosonic" bonding process, for example) also causes the formation of a capillary indentation (not shown) on the lead 12, the image of which can be confused with the image of the crescent 16.
Referring to FIG. 2, a top view of a crescent bond is shown. The wire 10 extends until it widens and flattens to form the crescent 16. The bottom of crescent 16 is defined to be the crescent base 18. The position of the crescent base 18 is herein defined to be a point at the intersection of a longitudinal axis 20 that bisects the crescent 16 and a line 22 that can be drawn so as to demark the beginning of the transition from the wire 10 to the crescent 16. Also shown are the crescent tips 21 and the crescent edge 23.
Referring to FIG. 3, the characteristics of the crescent 16 depend on several factors, primarily the capillary diameter 24, the capillary wall thickness 26, and the capillary edge angle 28. The wire diameter, and the force, temperature, and frequency of the capillary vibration also contribute to determining the characteristics of the crescent 16.
During the manufacture of packaged integrated circuits, it is necessary to inspect the crescent bonds that are formed on the leads of the lead frame. Ideally this inspection is performed immediately after the bonding step, so that errors in size, shape, placement, or lack of presence of the crescent bond can be detected and corrected before or soon after the error is repeated.
Commonly, inspection of the crescent bond is performed off-line by a skilled operator that manually inspects only a statistically significant sample of bonds using an optical microscope at high magnification with shallow depth of focus so as to measure bond size, shape, and presence/absence of crescent bonds. Only a sample of the bonds are inspected because the part must be removed from the production line to be inspected.
As wirebond machines increase in throughput rate, and as the geometries of semiconductor devices shrink, automated visual inspection becomes increasingly critical for inspection, and as well as for process control. By automating the crescent bond inspection step, it becomes possible to detect defective bonds at the earliest possible moment in the production process. Thus, bad parts can be removed before they are used, or before other testing is done, thereby reducing rework costs. Also, wirebonder equipment can be adjusted to correct the error before other bad parts are created, thereby minimizing waste, improving quality, and improving yield.
In many machine vision applications, a template for use with normalized correlation search, for example, can be developed from an ensemble of good images, e.g., good images of good crescent bonds. However, this approach does not provide a robust solution, apparently due to the high degree of variation in crescent bond shape and image contrast found in industrial application settings. Templates trained in this way that are used for finding the location and shape of crescent bonds do not yield satisfactory performance.